*2.1. Definition of PLES*

As early as 1984, some scholars in China put forward the dialectical relationship of PLE in the process of material exchange between human beings and nature [8]. Subsequently, the concept of PLE began to appear in research on sociology and ecology, and it was not until 2002 that PLE was gradually applied to research on geography [9]. At that time, research was guided only by the literal meaning of PLE, rather than the core of the

research, and did not give further explanations for its scientific connotations. Different scholars have discussed the definition of the basic connotation of PLES from different perspectives. Some scholars started from the top-level design of national planning and correlated PLES with main function zoning. These authors argued that ecological space mainly has ecological functions, accumulates ecological capital, and relates to the functions of production and living; production space focuses on the accumulation of production capital, and relates to living functions, which is equivalent to the key development zone and optimized development zone in the main functional area of the country; and living space mainly plays the service function of living, accumulates living capital, and gives play to the functions of both production and ecology. This understanding is equivalent to restricted development zones in the national main function area [10]. However, main function zoning is only stipulated at the national and provincial levels, whereas the basic evaluation unit is the county administrative region, which is unable to control smaller spatial areas, and lacks overall planning capabilities [11].

Wu [12] was the first to clearly define PLES from the perspective of the PLE function. He argued that ecological space refers to an area featuring important ecological functions, with ecological products and services as its main function; production space is an area with the main function of providing industrial products, agricultural products, and service products, primarily including industrial and mining construction space and agricultural production space; and living space is a region that provides the main function of human habitation and public activity, primarily including the urban and town settlement space and rural living space. Together, the three dimensions of production, living, and ecology constitute the whole national space. Zhu [13,14] enriched the above definition while also emphasizing the dominant function of land-use, arguing that production space is mainly used for production and operation activities. Living space provides places for people to live, consume, and engage in recreation, whereas ecological space is the sum of the environment needed or occupied by a species in a macroscopic stable state, and provides ecological products necessary for human beings.

Territorial space is a complex geographical system, and includes land, water, mineral resources, ecology, social economy, and other different resource elements; moreover, there are extremely complex interactions among the elements [15]. From this perspective, PLES covers biophysical processes; direct and indirect production; as well as spiritual, cultural, leisure, and aesthetic needs, and represents the product of the synergy and coupling of multiple systems of nature, society, and economy [16–18]. Among them, ecological space is the foundation that enables production space and living space to realize their own functions, and is key to coordinating the relationship between humans and land, and achieving regional sustainable development [19,20].
